The present invention relates generally to the field of lighting devices, and more specifically to devices capable of a low profile which utilize point sources, such as light emitting diodes, for illumination.
Low profile lighting devices are useful in a variety of applications, such as decorative strip lighting or display panel illumination. One common form of strip lighting utilizes neon tubes. Neon tubes have the advantages of being lightweight and lending themselves to decorative lighting. Further, the light output from neon tubes is relatively diffuse and uniform in appearance. However, neon tubes are fragile, require high voltage, and generate significant radio-frequency (RF) interference, which must often be shielded at significant cost. Fluorescent lighting is likewise diffuse, but is generally limited to short lengths and typically includes unattractive electrical connections.
Display panel illumination is often accomplished by placing light sources behind the panel to illuminate it. A diffuser box containing a light source can generate output that is fairly uniform, so long as the light sources are sufficiently far away from the display panel. This makes such devices bulky, however. If the light sources are too close to the panel, the illumination will no longer be uniform, and the sources will be seen as xe2x80x9chot spots.xe2x80x9d Thus there is a need for a compact lighting device that provides uniform output intensity.
According to one aspect of the invention, an illumination apparatus includes a cavity having reflective surfaces and an output area, as well as at least one light source disposed in the cavity, wherein the light source includes a point source and an optical diverter having a flared reflecting surface. The apparatus further includes an optical conditioning element over the output area, with the optical conditioning element including at least a diffuser, for example, a translucent film or plastic sheet. In a preferred embodiment, the flared surface is curved, and may be cuspated. In one preferred embodiment, the point source comprises an LED. In a preferred embodiment, the cavity reflecting surfaces are diffusively reflective.
According to another aspect of the invention, an illumination apparatus comprises a housing that includes a cavity having reflective surfaces and an output aperture. The apparatus also includes an optical conditioning element across the output aperture, in which the conditioning element comprises at least one sheet having a plurality of pixels. The apparatus further includes at least one light source disposed within the cavity directly beneath the conditioning element, in which the (at least one) light source includes a point source spaced less than 3-xc2xd inches from the sheet that illuminates the reflective surfaces such that the ratio of the luminance of adjacent pixels is between 0.95 and 1.05 and such that the ratio of the luminance of non-adjacent pixels is between 0.5 and 2.0, whereby the appearance of illumination at the sheet is substantially uniform. In a preferred embodiment, the optical conditioning element includes a diffuser sheet disposed below the prism sheet. In one preferred embodiment, the optical conditioning element includes a second prism sheet with orientation 90xc2x0 from the first. In a preferred embodiment, the (at least one) light source includes a point source and a total internal reflection lens having a cuspated surface for reflecting light from the point source against the diffusive reflective surfaces.
According to another aspect of the invention, an illumination apparatus includes a cavity formed by reflective material, in which the cavity has an output area. An optical conditioning element is at the output area. The apparatus further includes a light source in the cavity, in which the light source includes a point source and an optical diverter having a reflecting surface which is partially reflective and partially transmissive. The reflective surface of the diverter allows a portion of light incident thereon to pass through the reflecting surface, while reflecting another portion of the incident light onto the reflective material of the cavity. The reflective material reflects light within the cavity, whereby the output area and the optical conditioning element are illuminated. In a preferred embodiment, the reflecting surface of the diverter is comprised of scattering centers which scatter light incident thereon.
According to yet another aspect of the invention, an illumination apparatus includes an optical diverter. The diverter includes transparent material having a reflecting surface formed by a refractive index interface configured to totally internally reflect light from a point source positioned to emit a first portion of light rays towards the reflecting surface and a second portion of light rays towards a side surface of the diverter. The diverter includes a refracting interface that refracts the second portion of light rays towards the reflecting surface, such that both the first and second portions of light rays are reflected from the reflecting surface.
According to yet another aspect of the invention, there is provided an optical diverter that includes transparent material having a flared reflecting surface formed by a refractive index interface. The interface is configured to totally internally reflect light from a point source which is positioned adjacent to an apex of the flared reflecting surface and which emits light rays for reflection by the reflecting surface.
According to a further aspect of the invention, an illumination apparatus includes a cavity having reflective surfaces and an output area. The apparatus further includes at least one light source disposed in the cavity, in which the light source includes a point source and an optical diverter having a surface that is partially reflective and partially transmissive. The apparatus also includes an optical conditioning element over the output area, in which the optical conditioning element includes a diffuser, wherein the diverter is positioned between the point source and optical conditioning element such that (a) a portion of light emitted by the point source is reflected from the diverter towards the reflective surfaces of the cavity, and (b) another portion of light emitted by the point source is transmitted through the surface of the diverter towards the optical conditioning element, with the diverter sized to allow at least a substantial portion of the reflected light to reach the optical conditioning element without passing through the diverter, and wherein the diverter and the reflective surfaces of the cavity are arranged to allow at least a substantial portion of the transmitted light to reach the optical conditioning element without undergoing reflection.